数据库完整封装（增、删、查、改）

const Config = require("./config");

const mongodb = require("mongodb");
const MongoClient = mongodb.MongoClient;
const ObjectId = mongodb.ObjectId;

const client = new MongoClient(Config.dbUrl, { useUnifiedTopology: true });

class Db {
  // 解决多次实例化，实例不共享的问题
  static getInstance() {
    if (!Db.instance) {
      Db.instance = new Db();
    }
    return Db.instance;
  }
  constructor() {
    this.dbClient = "";
    this.connect();
  }
  connect() {
    return new Promise((resolve, reject) => {
      // 把数据库的连接变成长连接，解决多次连接数据库的操作
      if (this.dbClient) {
        resolve(this.dbClient);
      } else {
        client.connect(err => {
          if (err) {
            reject(err);
          } else {
            this.dbClient = client.db(Config.dbName);
            resolve(this.dbClient);
          }
        });
      }
    });
  }
  find(collectionName, json) {
    return new Promise((resolve, reject) => {
      this.connect()
        .then(db => {
          const collection = db.collection(collectionName);
          collection.find(json).toArray((err, docs) => {
            if (err) {
              reject(err);
            } else {
              resolve(docs);
            }
          });
        })
        .catch(err => {
          reject(err);
        });
    });
  }
  insert(collectionName, json) {
    return new Promise((resolve, reject) => {
      this.connect()
        .then(db => {
          const collection = db.collection(collectionName);
          collection.insertOne(json, (err, docs) => {
            if (err) {
              reject(err);
            } else {
              resolve(docs);
            }
          });
        })
        .catch(err => {
          reject(err);
        });
    });
  }
  update(collectionName, json1, json2) {
    return new Promise((resolve, reject) => {
      this.connect()
        .then(db => {
          const collection = db.collection(collectionName);
          collection.updateOne(
            json1,
            {
              $set: json2
            },
            (err, docs) => {
              if (err) {
                reject(err);
              } else {
                resolve(docs);
              }
            }
          );
        })
        .catch(err => {
          reject(err);
        });
    });
  }
  delete(collectionName, json) {
    return new Promise((resolve, reject) => {
      this.connect()
        .then(db => {
          const collection = db.collection(collectionName);
          collection.deleteOne(json, (err, docs) => {
            if (err) {
              reject(err);
            } else {
              resolve(docs);
            }
          });
        })
        .catch(err => {
          reject(err);
        });
    });
  }
  getObjectId(id) {
    return ObjectId(id);
  }
}

module.exports = Db.getInstance();